What is ALC on a preamp?

Question

I have a preamp rescued from an old stereo: BA3308

It has a pin "ALC" and the data sheet keeps going on about how splendid this ALC is and how it has a built-in ALC diode and such.

Nowhere does it tell you what ALC is, or how you should use this ALC pin.

I have looked for meanings of ALC online, and I have found 2 possible meanings that would fit this device:

Automatic Level Control
Audio Limiter Control

There may be others...

What is it? How should I connect it? is it an input, or an output? It's a major feature of this chip, and I guess it's assumed you know what it is as you chose the chip.

score 6 · Accepted Answer · answered Sep 13 '11 at 15:57

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If you look at the datasheet, there is an example application circuit:

enter image description here

Notice how the amplifier is used for amplifying for both recording and playback. The same tape head is also used for both functions.

ALC (automatic level control) was an important function in tape recorders, as the signal had to be the right level, strong enough to overcome noise but not too high to cause saturation. A variable-gain amplifier is a bit tricky to do without distortion, so having it integrated into the preamplifier was a convenience for the system designer.

I don't know what you plan to do with the part, but an obvious choice would be an audio compressor. It won't be super high-fidelity, but maybe good enough for a guitar stomp box.

answered Sep 13 '11 at 15:57

markrages

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I don't see that in the datasheet I linked to above - you must have a different one. Fancy posting a link to it? – Majenko Sep 13 '11 at 16:18
It is the ROHM datasheet form alldatasheet.com – markrages Sep 13 '11 at 16:21
You mean http://www.alldatasheet.com/datasheet-pdf/pdf/173415/ROHM/BA3308.html ? That's the one I linked to - it has no circuit in it. – Majenko Sep 13 '11 at 16:23
http://html.alldatasheet.com/html-pdf/35947/ROHM/BA3308/335/4/BA3308.html – markrages Sep 13 '11 at 16:28
Aha... they have it listed twice... Don't you just hate it when they do that? – Majenko Sep 13 '11 at 16:35
Actually, about the only thing I like about alldatasheet.com is the fact that I can sometimes find outdatet information when I want to. For real data sheets, I always go to the manufacturer, never to alldatasheet or the like. – zebonaut Sep 13 '11 at 20:09

score 2 · Answer 2 · answered Sep 13 '11 at 15:53

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If they talk all the time about ALC, and fail to explain what it is, all I can say then is that the datasheet stinks.

The application example on page 3 gives us some idea. The ALC signal is taken from the output, via a diode and an external capacitor. That means it's a peak detector, whose level is then used to control the amplifier's amplification. So that looks very much like Automatic Level Control, though Audio Limiter Control is a close relative, if not exactly the same.

answered Sep 13 '11 at 15:53

stevenvh

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this feature is more commonly referred to as "automatic gain control" or AGC in my experience... – vicatcu Sep 13 '11 at 16:07
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For reasons unknown, it is called ALC in tape recorders and AGC in radio receivers. – markrages Sep 13 '11 at 16:30
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It was "Automatic Volume Control" or AVC in radio receivers, at one time. – Leon Heller Sep 13 '11 at 17:12

zebonaut · Answer 3 · 2011-09-13T16:12:16.537

As the manufacturer says in this tech note, ALC means Automatic Level Control. There's a block diagram that looks like the ALC picks up the output signal and uses it to adjust integrated potentiometers at the input pins of the forward path.

It looks like the IC is intended to be used with cassette tape recorders. Cheapo recorders often don't have a potentiometer where one can set the recording level. Instead, an circuit tries to adjust the level of the signal that is to be recorded such that it is neither too silent and vanishes in the tape noise, nor too loud so it would cause tape saturation (distortion).

The text next to Fig. 15 explains how to use the feature. Basically, there's an RC combination that allows you to set how fast the IC will react to varying levels of the signal.

score 0 · Answer 4 · answered Jun 09 '12 at 14:35

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See this one: AGC- Automatic Gain Control It mean variable dB on amplifying.

ALC- Automatic Level Control It mean limitation of Peak.

answered Jun 09 '12 at 14:35

Tuvshin

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Not really helpful in answering the question. – Dave Tweed Oct 19 '12 at 21:58

score -1 · Answer 5 · answered Jul 08 '23 at 20:25

If that ALC determines the level of playback as well as recording (although I fail to see what Automatic Level Control would be important for playback as the main concern would be the noise level and saturation level inherent in the recording tape formulation: once it's recorded, neither can one go back and fix it using a simple full-spectrum attenuator or gain stage)... that could explain why a Bell & Howell portable tape deck I have suddenly stopped emitting sound from recorded tapes (well, it was extremely faint, to the point that you had to strain to even know that something was being played), but now is playing normally today with only a slight touch to the ALC on/off switch. I'll temporarily place the blame on a faulty ALC circuit which somehow became overloaded and caused the rest of the circuitry to seriously attenuate any signal coming from the tape deck. Thanks for the (perhaps unintentional) insight!

(Yes, I did study Electrical Engineering and graduated, but I rarely feel compelled to open up a machine and see if I can fix things, unless I absolutely really have to; most of the time, if I can get it done inexpensively by a licensed tech, I will. Did I happen to mention I didn't pursue a career in Electrical Engineering after graduation?)

What is ALC on a preamp?

5 Answers5